For more precise information, please inform us what device you are using but in the meantime try the following:
First make sure that auto sync is turned on.
Go to Settings >Tap Accounts >Tap Google>Tap the account name (email address)>Tap the appropriate data synchronization options (e.g. Sync Gmail etc.)> click on to enable.

Tap Email App > From an Inbox, tap More (located in the upper right)>Tap Settings>Tap your email account> Tap Sync account switch to turn on On indicator> Then tap Sync schedule >Edit any of the following: Note To edit, tap the option then select the desired setting.> Set sync schedule > Sync data While roaming

If you wish, you can also make sure that sync is enabled when you are connected to data.
Go to Settings >Tap Data Usage >Tap Menu icon>Put a check mark on Auto Sync Data option.